I tried to follow Hilke's comment about trying to brighten my pine warbler picture. I only have Microsoft Photo Editor on my computer, so maybe this is the reason why the results were not that impressive. Or maybe it's just that I need to actually read a little bit more about pictures processing.
Update : Hilke was kind enough to process the picture for me. Here is what she was able to obtain:
Last week was relatively sunny and warm, and I took this picture of a song sparrow. Easy shot, as the sparrow was basically posing in the morning sun. That's SOO much easier with good sun light!!!!